1. Technical Field
Some high molecular weight polymers, such as, for example, polybutylene terephthalate (PBT) polymers, polyethylene terephthalate polymers (PET), and copolyetheresters, are synthesized from the product obtained from the direct esterification of a diacid, such as terephthalic acid, with a glycol, such as 1,4-butanediol or ethylene glycol. The direct esterification reaction has been known to take as long as 3 to 4 hours to complete. This long reaction time, usually at high temperatures, is undesirable not only because of the length of the processing time required, but also because the likelihood that side reactions will occur increases with time. A particularly undesirable side reaction that can occur is the conversion of hydroxybutyl end groups into tetrahydrofuran (THF). Hydroxybutyl end groups are generated when the acid ends of a diacid are esterified with 1,4-butanediol. A faster reaction time is thus desired in order to reduce the amount of THF generated during a direct esterification reaction between a diacid and 1,4-butanediol.
The present invention is related to significantly increasing the direct esterification reaction rate of a diacid with a glycol, particularly with 1,4-butanediol or ethylene glycol. More specifically, the present invention is a process wherein greater than 95% of the acid ends of a diacid are directly esterified by a glycol, particularly by 1,4-butanediol or ethylene gylcol, in less than seventy minutes. Further, when 1,4-butanediol is the glycol used in a direct esterification reaction, the present invention provides a process whereby less than 5% of the 1,4-butanediol is converted to THF during said reaction. The product of this process can be polycondensed to form a high molecular weight polymer, such as PBT or PET, or it can be further reacted to produce other types of high molecular weight polymers, such as, for example, copolyetheresters.
2. Description of Related Art
U.S. Pat. No. 4,680,376 discloses a process for the continuous production of PBT by the direct esterification of terephthalic acid with 1,4-butanediol in the presence of tin- or titanium-containing catalysts, followed by polycondensation of the direct esterification product. The direct esterification reaction must be conducted under pressure and at temperatures between 225.degree. C. and 260.degree. C. The molar ratio of 1,4-butanediol to terephthalic acid is 2 to 3.8:1. There is no example showing that greater than ninety-five percent of the acid ends of a diacid are directly esterified in less than seventy minutes at atmospheric pressure.
East German Patent 241,826 discloses a process for preparing PBT by esterification of terephthalic acid with butanediol, followed by subsequent polycondensation. The esterification reaction times disclosed are at least 90 minutes and butanediol losses via conversion to THF are reported at 6-11 percent. Further, the reference is focused on increasing the polycondensation reaction rate, not on increasing the direct esterification reaction rate.
U.S. Pat. No. 4,049,635 discloses a process for preparing a polyester by transesterifying 1,4-butanediol and at least one dialkylbenzene dicarboxylate in the presence of a tetraalkyl titanate catalyst and then polycondensing the transesterification reaction product to high molecular weight polymer. The transesterification reaction is conducted over a period ranging from 75 minutes to 3 hours.
British patent specification 1,320,621 discloses a process for the production of fiber-forming poly(tetramethylene terephthalate) comprised of the following three stages: (1) direct esterification, (2) vacuum letdown, and (3) polycondensation. The direct esterification step occurs over a 2 to 8 hour period, under pressure.
Japanese kokai patent no. 76-37187 discloses a method for manufacturing polyester which involves esterifying terephthalic acid and 1,4-butanediol and then subsequently polycondensing the esterification product to form polyester.
Japanese kokai patent no. 87-225521 discloses a method for manufacturing PBT which involves an esterification reaction, said esterification reaction being between a dicarboxylic acid component and a glycol component. The molar ratio of 1,4-butanediol to terephthalic acid should not exceed 1.5:1.
Although each of the above references disclose a process for esterifying a diacid, none disclose the particulars of the present invention. Further, for those cases where the glycol used is 1,4-butanediol, there is no process disclosed wherein less than 5 weight percent of the 1,4-butanediol used in the reaction is converted to THF.